The present invention relates to vibration monitoring and more particularly to monitoring the stimulation in any ultrasonic generator.
Vibration monitoring is useful in multiple systems and industries. Ultrasonic generators, including ultrasonic cleaners, ultrasonic welders, ultrasonic machining, and continuous ink jet drop generators, are used for a variety of purposes. For example, in order to provide precise charging and deflection of drops in a continuous ink jet printer, it is important that the drop break-up process produce uniformly sized and timed drops. Drop generators for such printers produce the required drop formation by vibrating the orifices from which the ink emerges.
Feedback transducers have been utilized for control of the stimulation amplitude and for tracking the resonance of the drop generator as discussed in U.S. Pat. No. 5,384,583, totally incorporated herein by reference. These feedback transducers work appropriately when the feedback signal has sufficient signal to noise. The use of a push-pull feedback system as discussed in that disclosure can effectively suppress noise due to charging transients or due to electronic coupling from the stimulation drive signal.
The individual transducers can be placed close to each other so that the noise picked up by the two transducers are similar, allowing the noise to be canceled. Proper placement of the individual transducers can help suppress output signals from extraneous vibrational modes.
However, for some drop generator designs, it is not practical to place the transducer appropriately to suppress all the other extraneous modes. This might be a result of insufficient space to place the feedback transducers, or low output amplitudes on available surface space. For some drop generator designs, to effectively suppress the detection of extraneous modes would require placement of feedback transducers in the space already occupied by the drive transducers. This results from the need to place drive transducers in a particular pattern to suppress the exciting of undesirable modes.
For such systems it would be desirable to employ the driving transducers as feedback transducers as well. While U.S. Pat. No. 3,868,698 makes use of the drive transducer impedance characteristics to track resonant frequency, it does not teach a means to monitor the vibration amplitude and phase for use in the control of the ink jet system.
It would be desirable to have an effective means to employ the piezoelectric drive crystals for both driving the drop generator and detecting the resulting vibration. Additionally, the large capacitance of piezoelectric drive transducers, when operated at high frequencies, can provide significant loading to the drive electronics. This can significantly limit the maximum drive amplitudes. It would, therefore, be desirable to have a means to allow for higher drive amplitudes, even with large capacitance levels of drive transducers.
The present invention provides a means, such as a circuit, which uses the driving piezoelectric transducers to monitor the induced vibration or stimulation in an ultrasonic generator, such as the drop generator of an ink jet printing system. The present invention finds utility not just in the field of ink jet printing, but in other fields including monitoring ultrasonic cleaners and welders.
In accordance with one aspect of the present invention, a differential circuit is used to compare the current to the drive transducers to a matched reference circuit. With the capacitive current from the piezoelectric transducer canceled out in this manner, the resulting output current provides a direct measure of the vibration amplitude of the drop generator. By adding an appropriate inductor in parallel to the capacitive piezoelectric drive transducers, the loading of the drive electronics is significantly reduced.
Other objects and advantages of the invention will be apparent from the following description and the appended claims.